Machine guns, such as the Army M60 family, are gas operated and require gas pressure generated when a cartridge is fired to recycle the gun for loading and firing the next cartridge. This gas pressure is built up behind the projectile before it leaves the gun barrel and becomes the pressure source for the subsequent movement of operating parts.
Firing blank ammunition is often done in troop training to provide realism to the training. For example, machine guns firing blanks are trained on our own troops as they advance on enemy installations in war games and other training exercises. With blank cartridges there is no bullet leaving the gun barrel behind which gunpowder generated gases are pressurized and, without some other impedance to the gases, they simply pass out of the barrel. There then is no pressurization of the gases to ready the gun for firing the next round.
To achieve the necessary gas pressurization while still using blanks, a prior art adapter has been devised for restricting the gas flow from the end of the barrel. This adapter has a restrictor tube that fits within the flash suppressor at the end of the barrel and is held in position with a sheet metal bracket clamped to the vertical post of the front sight of the gun.
The foregoing adapter has several defects. It required precise machining to fit snugly into the flash suppressor. After short use gas leaks developed between it and the end of the gun barrel. Since it consisted of several parts, it was costly to make and assemble. It also had a safety hazard in that if a live round of ammunition was accidentally fired, it shattered into laterally projected shrapnel which became a potential danger to the operator. Since this adapter is detachable from the gun as a practical matter in the field, many get lost and have to be replaced.